The invention relates to a safety belt roll-up mechanism having a belt shaft that is mounted in a housing and serves as a carrier for a belt strap that can be wound onto and unwound from the belt shaft, whereby the belt roll-up mechanism is provided with a force limitation device that operates in a multi-stage manner and has a torque rod and a switching device that couples the torque rod, in various clamping lengths, with the belt shaft, and whereby the extent of the belt strap unwound from the belt shaft is utilized for the actuation of the force limitation device.
A safety belt roll-up mechanism having the aforementioned features is known from DE 27 27 470 A1. The known safety belt roll-up mechanism has a torque rod as a force limitation device that at its one end is fixedly connected to the shaft blocking mechanism, and at its other end is coupled to the belt shaft in such a way that even when the shaft blocking mechanism is triggered, the belt shaft, upon torsion of the torque rod, can still rotate further by a certain amount. Since the adjustment of the force limitation level should be effected as a function of, in particular, the size of a vehicle occupant, the known belt roll-up mechanism has a device for the continuous alteration of the clamping length of the torque rod, which clamping length, in one embodiment of the known belt roll-up mechanism, is to be controllable as a function of the length of the belt strap withdrawn by connecting a sliding sleeve, which fixes the clamping length, with the belt shaft via a gear mechanism.
Reference is further made to U.S. Pat. No. 6,616,081 (hereinafter “'081”), the disclosure of which is incorporated herein by reference. This patent discloses a known two-stage, force-limiting device, as well as a pyrotechnically operated force limiting device which is utilized in the present invention, as described below in detail.
According to the '081 patent, the control of the switching to the different clamping lengths of the torsion rod takes place in that the release of the belt band, caused by the effect of the force limiting device of the seat belt roll-up mechanism, is used as the gauge for ignition of the pyrotechnical switching device (FIG. 1 of '081). In this connection, a measuring device 37 is provided on the belt roll-up mechanism 10, for example in the form of a potentiometer, which determines the speed of the shaft rotations or the angular speed. If the measured value exceeds a threshold, this is identified as the beginning of the force limiting process, and the further belt release is determined via the potentiometer. Thus, '081 is based on a switching time point, which is provided or constructively determined during the occurrence of an accident, for example during the force limiting process.
The known safety belt roll-up mechanisms have the drawback that the sliding sleeve does not represent an adequate reliability for the fixing of the clamping length of the torque rod, and in addition, the mechanics for a continuous adaptation of the clamping length of the torque rod to the rotational speed of the belt shaft, and hence to the respectively existing belt strap withdrawal, is complicated.